Images of objects comprising various types of materials can be generated using X-ray scattering. The intensity of scattered X-rays is related to the atomic number of the material scattering the X-rays. In general, for atomic numbers less then 25, the intensity of X-ray backscatter, or X-ray reflectance, decreases as the atomic number increases. Objects are potentially visible in X-ray images of human subjects due to the difference in X-ray reflectance between the objects and human tissue. Thus, conventionally, images are primarily modulated by variations in the atomic number of the subject's body.
Non-metallic objects are commonly composed of low atomic number elements similar to those of human tissue, i.e. hydrogen, carbon, nitrogen, and oxygen. Soft human tissue scatters a significant amount of X-rays due to the relatively low atomic number of hydrogen, carbon, and oxygen in relatively high concentration. Due to the high atomic number of calcium, bones near the surface of the body, comprised mainly of calcium, produce much less scatter. Concealed objects, especially metals, can be easily visualized in the images due to their significant difference in atomic composition from the background of human tissue.
In conventional systems, especially of the X-ray transmission type, an operator is required to identify very low contrast objects in the presence of inherent image clutter that results from the imaging of internal human anatomy. The difficulty of this task results in poor detection capability for a wide range of dangerous objects composed of low atomic number elements, such as plastics or ceramics, which are often masked by the low atomic number elements which comprise the human body.
Radiation exposure limit is also an important consideration in X-ray concealed object detection systems. Various prior art X-ray systems for detecting objects concealed on persons have limitations in their design and method that prohibit them from achieving low radiation doses, or prevent the generation of high image quality, which are prerequisites for commercial acceptance. An inspection system that operates at a low level of radiation exposure is limited in its precision by the small amount of radiation that can be directed towards a person being screened.
X-ray absorption and scattering further reduces the amount of X-rays available to form an image of the person and any concealed objects. In prior art systems, this low number of detected X-rays has resulted in unacceptably poor image quality.
As shown in FIG. 1a, conventional systems that employ low-radiation techniques for generating images of individuals 101 do so without suppressing sensitive anatomical features 105, thereby allowing for the visual display of personal anatomical details 105 and invading the privacy of people being screened. Also, in existing systems, detailed images are produced by characteristics of the subject's body and any object concealed under the subject's clothing. The system operator then inspects each image for evidence of concealed objects. The equipment operator and security personnel responsible for analyzing conventional images are thus privy to these personal anatomical details.
In addition, current systems that are used to enhance or improve upon levels of privacy in resultant X-ray images of humans use basic brightness, contrast adjustment and subsequent manipulation to wash out body contours and detail or highlight metallic or inorganic objects. Thus, threat items 125b tend to appear as a darker contrast than the body 135b, as shown in FIG. 1b. For example, FIG. 1b illustrates different X-ray images generated from brightness and contrast manipulations at low 110b, medium 115b, and high privacy 120b settings, respectively. While this prior art method is effective in screening for inorganic threat items (i.e. metal objects) 125b, details of organic threat items (i.e. explosives or contraband) are lost in the image processing.
Further, conventional systems tend to employ complicated image processing methods on the entire image, such as the use of edge algorithms, smoothing filters or even object recognition. While these methods work well for inorganic threat items at a high privacy level and even offer reasonable results for organic objects at low privacy settings, they remove essential information that could be displayed without decreasing privacy levels. For example, FIG. 1c illustrates a series of X-ray images generated from complicated image processing methods, at low 110c, medium 115c, and high privacy 120c settings, respectively. Note that, at higher privacy levels, while the general outline of the body 135c and metal object 125c still appear, details around the feet 145c and fingers 150c begin to disappear.
It should be noted herein that conventional image processing techniques for protecting privacy, as shown and described in FIGS. 1a, 1b, and 1c above, tend to diminish non-body images as well, and thus, degrade the image presented to the viewer. For example, but not limited to such example, employing a traditional combination of increased brightness and contrast to diminish anatomical features may also result in the washing out of smaller and thin threat objects, such as plastic explosives, because they have properties similar to human skin. Further, another traditional processing technique employs blurring of anatomical detail, which also results in the reduced threat detection.
Additionally, objects that are located “off-body” are difficult to image, given the darker background of air or a conventional screening back-drop. When a filter is applied to the resultant images, using conventional image processing methods, almost all objects that are at the person's side or located inside of loose clothing tend to disappear.
Still further, more advanced methods include using edge detection algorithms, which effectively enhance and apply a threshold calculation for adjacent pixels. If the contrast between adjacent pixels is great enough, then the deviation will be displayed. Most body parts, however, protrude in such a way that deems the edge partial, and appearing on only the bottom half of the object. It is possible to use a filtering algorithm to suppress objects with a partial low edge. However, this may result in the inadvertent omission of threat detection, because the threat could be imaged in such a way that only a partial edge is detected and thus the threat is filtered out.
Still further, previous techniques have been employed that sequentially apply processing techniques to images. These techniques use threshold values for pixel brightness or contrast to make objects easier to distinguish or filter them out entirely.
Still further, some scanning systems employ techniques of artificial intelligence to recognize body parts, such as but not limited to shoulders, head, feet, and identify them in the image. The computer system can then use triangulation techniques to roughly calculate and estimate where the common private parts are and blur these regions, or filter only those areas. Still further, correlation algorithms can be used that compares known shapes of objects, such as a gun or human anatomy, and enhance or filter them, respectively. For example, if a gun is detected, a correlation algorithm can be used to associate the imaged shape with a gun and subsequently enhance the image.
As detailed above, prior art systems fail to display only the suspect region(s) in detail while suppressing anatomical details or other features of the human body that are not required for analysis. Thus, the prior art does not provide a system that achieves the correct balance between the conflicting, yet equally important principles of maximizing security and inspection capability while retaining a sufficient level of privacy for the person under inspection.
What is therefore needed is a processing technique that allows for different regions of an original image to be processed separately so that different portions of the image can be optimized to desired object detection or privacy levels.
What is also needed therefore is an image processing technique that allows for maximum threat detection performance with minimum display of anatomical details.
What is also needed is an X-ray inspection system that takes advantage of image processing techniques to allow for maximum threat detection performance with minimum display of anatomical details.
What is also needed, therefore, is an X-ray inspection system for detecting concealed objects carried by a person, capable of detecting low atomic number (“low-Z”) materials as well as metals, but which does not expose the subject to radiation doses significantly higher than normal environmental radiation levels.